Which instrument is primarily used to determine an aircraft's heading?

The instrument primarily used to determine an aircraft's heading is the magnetic compass. The compass provides pilots with real-time information about the aircraft's orientation relative to magnetic north. This is crucial for navigation, as maintaining the correct heading is essential for reaching a desired destination, especially when flying cross-country.

While other instruments play significant roles in flight, they do not directly provide heading information. The altimeter is used to measure altitude, the vertical speed indicator indicates whether the aircraft is climbing or descending, and the airspeed indicator shows the speed of the aircraft relative to the surrounding air. Therefore, none of these instruments can replace the compass in its specific function of indicating heading.